EF 50/1.8 AF Experiment?

Wilba wrote:
David J Taylor wrote:
Wilba wrote:

Any thoughts about why the 50/1.8 figures make sense (farside focus...
0.8mm outside the DOF), but it doesn't work like that for the 18-55?

As others have said, the focal point will shift with f/number,

How is that a factor when shooting at widest apertures?

Because with a specific lens and AF sensor the focusing takes place
always at the effective aperture of the AF sensor, whereas the
aperture with which the photograph is taken can vary. Many cameras
have a max AF sensor aperture of around f6. Some go down as far as
f2.8. Hence if you're using a lens at f1.8, and it happens to be a
spherical lens design with aperture related focus drift (as many of
the golden oldie 50mms are), then this is an important factor.

and the focus sensors may only accept the smaller cone of rays rather
than the full f/1.8 cone.

I don't understand any of that. :- )

It means the AF sensor has a smaller effective aperture than is being
used to take the photograph. If you want the best results from a wide
aperture 50mm of spherical design then you need to understand
this. Alternatively you could buy a modern aspherical design. One of
the great benefits of modern technology is that it enables people who
don't understand what they're doing to do it well.

--
Chris Malcolm

Wilba wrote:
Paul Furman wrote:
Wilba wrote:
Wilba wrote:
Wilba wrote:
Doug McDonald wrote:
You take pictures using manual focus (at f/1.8) with the focus
indicator (the little dot at lower right in my 30D) as the criterion
of correct focus. Try to get it centered in the middle of
the "in focus is indicated" range. Make several tries. If this
gives better focus than real autofocus, something is wrong with the
"focus movement prediction" system.

My results say that the indicator is better than real autofocus,
but not by a lot. This is using only the center spot on text
7 feet away.
Ah, this is exactly the kind of idea I need! That experiment attempts
to separate the detection (AF sensor) and actuation (lens motor)
functions within the system. I'll have a play with it today. Thanks a
bunch.
Well, it sure has been an interesting day. :- )

Here are some preliminary results with a 45 degree target. I need to
repeat with a different target before I would state the following with
confidence, and it could be quite different for other units depending on
their calibration. But anyway...

Sharpness in the very centre at f/1.8 appeared fine, practically
indistinguishable from f/2.8 or f/4. In previous tests I might not have
looked hard enough at the very centre of the image, and may have been
misled by other sources of variation. So that's suggests a disproof of
my "soft at f/1.8" theory.

I found that my phase detect AF sensor has sidedness. If I start with
the lens focussed closer than the subject, the results are uniformly
excellent, whether autofocussing or manually focussing using the AF
confirmation (as Doug described above).

If I start with the lens focussed behind the subject, and I manually
focus using the AF confirmation, focus is always off by the same tiny
amount (one click towards infinity in the EOS Utility will bring it into
optimal focus).

With initial focus behind the subject and PD autofocus, about seven
shots out of ten are out by the same one click as the manual focus, and
the rest are optimal, like when starting from the nearside. I assume
that the good ones come about from the lens overshooting and then the
system corrects towards infinity (so it ultimately approaches focus from
the nearside).

With an external aperture which gives similar exposures to f/2.8 (with
the lens at f/1.8), focus is still excellent starting from the near
side, and it improves the performance when starting from the far side
(but still not optimal).

Those results, and the fact that the lens is able to focus perfectly via
contrast detect, suggest to me that the "crude and sloppy mechanism"
theory is bogus. I suspect the truth is about the performance of the PD
AF sensor with this lens, specifically how much latitude it has for
confirming a focus. I noticed when manually focussing with a macro rail
that I could move the camera quite some (micro) distance and still get a
focus confirmation beep.
I've been thinking a lot about the width of the "beep band" and what it
means, so I measured it with a macro rail. With the lens at its closest
focus (430mm from the sensor), the far beep is 3.0mm from the near beep.
A conventional DOF calculation gives around 4.3mm, and a focus from the
nearside appears to put the subject bang in the middle of that. So that
means a farside focus would put the subject something like 0.8mm outside
the DOF, which sounds about right.
|4.3mm| theoretical
-|3mm|- measured
.8mm
- 430mm - [camera]

I also tried it at that distance with an EF-S 18-55 at about 50mm
(f/5.6). The beep band is 12mm, the DOF is 13mm, and where you start from
makes very little difference to the focus achieved (although from the
nearside is best again). I wish I had another wide-aperture FFL lens to
compare.
|13mm| theoretical
-|12 |- measured

Any thoughts about why the 50/1.8 figures make sense (farside focus...
0.8mm outside the DOF), but it doesn't work like that for the 18-55?
Did I get those diagrams right?

Um, I don't understand your diagrams. :- ) Let me try again.

Your follow-up post diagram makes sense.
You have the camera & subject reversed from mine and enough room to show
things with more detail.

I see my diagrams were mistaken so ignore.


With f/1.8 and a subject in optimal focus at 430mm, a conventional DOF
calculation gives 4.3mm, so lets assume 2.2mm each side (I know we can't
rely on this figure, but it's not a million miles from what I see). Starting
with the camera at 425mm and moving it away from the subject until it beeps
focus confirmation, we get optimal focus again at 430mm. Starting with the
camera at 435mm and moving in, it beeps at 433mm, which is 0.8mm outside the
theoretical DOF and is clearly out of focus.

I'm really not following what 'makes sense' about the f/1.8 error.

What makes sense is that a focus 3mm beyond the optimal camera to subject
distance would put the subject outside the DOF (3.0 - 4.3/2 = 0.8).

Are these distances confirmed with photo results in a focus test
pattern?

Yes, the above is just a mathematical expression of what I'm seeing from
doing the beep tests. When I said, "a farside focus would put the subject
something like 0.8mm outside the DOF, which sounds about right", I meant
that the numbers agree with what I'm seeing - the subject is definitely a
small distance outside the DOF in applicable images.

What's interesting is that the beep tests give the same images as AF. That
tells me that the focus errors that people see with this lens when focussing
are probably not due to a lack of precision in the lens's focus mechanism
(as is commonly believed), but may be due to the fact that the focus sensor
will confirm a focus when the subject is clearly not in focus.

Excellent.


That happens
100% of the time when I focus by moving my camera, and something in the
order of 70% of the time when I AF, with the subject further away than where
the lens is initially focussed. (YMMV with a differently calibrated body and
lens.)

For one thing, I wouldn't bother with calculated DOF, especially for close
up.

Right, that's just to have some numbers that fit with what I'm seeing.

I would predict results kinda like this due to the AF sensors getting a
restricted aperture view of things with a bit more DOF:

I don't follow that, sorry. (All tests and calculations were done at widest
aperture - f/1.8.)

But without stopping down, I can't explain the difference from behind or
in front... however, that's at least a useful conclusion from all this: if
you start focused from in front, better focus can be achieved. So maybe
that's the only relevant lesson from all this, and it's a good one.

Yes. :- ) (Again, results may be quite different with other combinations
of bodies and lenses.)

OK, here's a possible explanation for the front/back difference. The
character of the out of focus area in front & back is indeed different. In
terms of bokeh, the background is usually smoother and the foreground
harsh. There are a few specialized 'defocus control' lenses which have an
extra ring to change this relationship. I'm not sure quite how that would
effect the AF confirm but it could be a factor.

Very interesting idea, thanks very much. This -
http://www.stacken.kth.se/~maxz/defocuscontrol/ (and the links in it to
http://www.luminous-landscape.com/essays/bokeh.shtml) - gave me enough to
get what you're saying.

That brings us back again to the notion that it is the optical performance
of the 50/1.8 that causes the erratic focus from the farside, not the
perceived crudeness of the focus mechanism.

What's different for me now is that I believe the erratic farside focus is
not about the sharpness wide open, but more likely about the different
"character of the out of focus area" on the nearside and farside of the
plane of focus.

Yeah, that's the only thing that makes sense, though it's still a bit
sketchy. In the middle of this page:
http://www.stacken.kth.se/~maxz/defocuscontrol/
the crops of dark blurry blobs show harsh rimmed OOF circles and perhaps
the AF sensor is grabbing those edges. I don't think that's quite right
but the best I can come up with.


--
Paul Furman
www.edgehill.net
www.baynatives.com

all google groups messages filtered due to spam

Chris Malcolm wrote:
Wilba wrote:
David J Taylor wrote:
Wilba wrote:
Any thoughts about why the 50/1.8 figures make sense (farside focus...
0.8mm outside the DOF), but it doesn't work like that for the 18-55?
As others have said, the focal point will shift with f/number,

How is that a factor when shooting at widest apertures?

Because with a specific lens and AF sensor the focusing takes place
always at the effective aperture of the AF sensor, whereas the
aperture with which the photograph is taken can vary. Many cameras
have a max AF sensor aperture of around f6. Some go down as far as
f2.8. Hence if you're using a lens at f1.8, and it happens to be a
spherical lens design with aperture related focus drift (as many of
the golden oldie 50mms are), then this is an important factor.

Yeah, this could still be the explanation. Which direction does the
focus shift when stopping down? -that would provide a clue.
http://diglloyd.com/diglloyd/free/FocusShift/index.html
"With every lens I’ve tested to date, the focus moves farther away. For
example, if focus at f/1.4 is centered at 1.00 meters, then by f/2.8 it
might now be centered at 1.02 meters."

On average (ignoring front/back approach), Wilba's camera is focusing
closer (when 'stopped down' through obstructions to the AF sensor), then
opening fully to the sensor for taking the pic; the actual point in
focus is further back. That's the opposite of what this effect would
suggest.

Theoretically the camera could correct for this given that it has the
lens data from the cpu connection. It is my understanding that Nikons
correct exposure for vignetting when stopping down and this is the
rational for not allowing entry level Nikon bodies to meter without a
cpu chip telling the camera which lens it has, where the pro models
allow inputing the lens specs manually.


and the focus sensors may only accept the smaller cone of rays rather
than the full f/1.8 cone.

I don't understand any of that. :- )

It means the AF sensor has a smaller effective aperture than is being
used to take the photograph. If you want the best results from a wide
aperture 50mm of spherical design then you need to understand
this. Alternatively you could buy a modern aspherical design. One of
the great benefits of modern technology is that it enables people who
don't understand what they're doing to do it well.



--
Paul Furman
www.edgehill.net
www.baynatives.com

all google groups messages filtered due to spam

"Wilba" wrote in message
...
David J Taylor wrote:
Wilba wrote:

Any thoughts about why the 50/1.8 figures make sense (farside focus...
0.8mm outside the DOF), but it doesn't work like that for the 18-55?

As others have said, the focal point will shift with f/number,

How is that a factor when shooting at widest apertures?

and the focus sensors may only accept the smaller cone of rays rather
than the full f/1.8 cone.

I don't understand any of that. :- )

See the ray diagram he
http://en.wikipedia.org/wiki/Spherical_aberration

With spherical aberration, for example, rays from the edge of the lens
(i.e. full aperture) focus at a different point from those from the middle
part of the lens, so the focus "point" can shift with aperture. The focus
sensors tend to have a narrower acceptance angle than f/1.8, hence they
will adjust the lens so that rays from nearer the centre will be focussed,
leaving the outer rays focussing at a different, incorrect position.

David

"Wilba" wrote in message
...
Wilba wrote:
Wilba wrote:
Doug McDonald wrote:

You take pictures using manual focus (at f/1.8) with the focus
indicator (the little dot at lower right in my 30D) as the criterion
of correct focus. Try to get it centered in the middle of
the "in focus is indicated" range. Make several tries. If this
gives better focus than real autofocus, something is wrong with the
"focus movement prediction" system.

My results say that the indicator is better than real autofocus,
but not by a lot. This is using only the center spot on text
7 feet away.

Ah, this is exactly the kind of idea I need! That experiment attempts to
separate the detection (AF sensor) and actuation (lens motor) functions
within the system. I'll have a play with it today. Thanks a bunch.

Well, it sure has been an interesting day. :- )

Here are some preliminary results with a 45 degree target. I need to
repeat with a different target before I would state the following with
confidence, and it could be quite different for other units depending on
their calibration. But anyway...

Sharpness in the very centre at f/1.8 appeared fine, practically
indistinguishable from f/2.8 or f/4. In previous tests I might not have
looked hard enough at the very centre of the image, and may have been
misled by other sources of variation. So that's suggests a disproof of my
"soft at f/1.8" theory.

I found that my phase detect AF sensor has sidedness. If I start with the
lens focussed closer than the subject, the results are uniformly
excellent, whether autofocussing or manually focussing using the AF
confirmation (as Doug described above).

If I start with the lens focussed behind the subject, and I manually
focus using the AF confirmation, focus is always off by the same tiny
amount (one click towards infinity in the EOS Utility will bring it into
optimal focus).

With initial focus behind the subject and PD autofocus, about seven shots
out of ten are out by the same one click as the manual focus, and the
rest are optimal, like when starting from the nearside. I assume that the
good ones come about from the lens overshooting and then the system
corrects towards infinity (so it ultimately approaches focus from the
nearside).

With an external aperture which gives similar exposures to f/2.8 (with
the lens at f/1.8), focus is still excellent starting from the near side,
and it improves the performance when starting from the far side (but
still not optimal).

Those results, and the fact that the lens is able to focus perfectly via
contrast detect, suggest to me that the "crude and sloppy mechanism"
theory is bogus. I suspect the truth is about the performance of the PD
AF sensor with this lens, specifically how much latitude it has for
confirming a focus. I noticed when manually focussing with a macro rail
that I could move the camera quite some (micro) distance and still get a
focus confirmation beep.

I've been thinking a lot about the width of the "beep band" and what it
means, so I measured it with a macro rail. With the lens at its closest
focus (430mm from the sensor), the far beep is 3.0mm from the near beep. A
conventional DOF calculation gives around 4.3mm, and a focus from the
nearside appears to put the subject bang in the middle of that. So that
means a farside focus would put the subject something like 0.8mm outside
the DOF, which sounds about right.

I also tried it at that distance with an EF-S 18-55 at about 50mm (f/5.6).
The beep band is 12mm, the DOF is 13mm, and where you start from makes
very little difference to the focus achieved (although from the nearside
is best again). I wish I had another wide-aperture FFL lens to compare.

Any thoughts about why the 50/1.8 figures make sense (farside focus...
0.8mm outside the DOF), but it doesn't work like that for the 18-55?


Keep in mind that according to Canon, AF accuracy is within 1 CoC for
non-pro EOS camera's and within 1/3 CoC for pro EOS camera's.
(About +/- 0.02 mm resp. +/- 0.010mm at the sensor)

On 31/12/2009 02:52, Chris Malcolm wrote:
Many cameras
have a max AF sensor aperture of around f6. Some go down as far as
f2.8.

Actually AFAIK most DSLRs have at least one dual AF sensor, that
switches between one sensor for lenses opening at least at f/5.6 and one
for lenses doing f/2.8 or better. The accuracy of the f/2.8 sensor is
required to focus within the DOF of the more open lenses, and
entry-level cameras lacking these dual sensors cannot use the more open
lenses effectively.


--
Bertrand

Chris Malcolm wrote:
Wilba wrote:
David J Taylor wrote:
Wilba wrote:

Any thoughts about why the 50/1.8 figures make sense (farside focus...
0.8mm outside the DOF), but it doesn't work like that for the 18-55?

As others have said, the focal point will shift with f/number,

How is that a factor when shooting at widest apertures?

Because with a specific lens and AF sensor the focusing takes place
always at the effective aperture of the AF sensor, whereas the
aperture with which the photograph is taken can vary. Many cameras
have a max AF sensor aperture of around f6. Some go down as far as
f2.8. Hence if you're using a lens at f1.8, and it happens to be a
spherical lens design with aperture related focus drift (as many of
the golden oldie 50mms are), then this is an important factor.

I thought aperture related focus drift happened if the shot was at an
aperture other than wide open. You're saying when I focus and shoot at the
lens's widest aperture, that's when I get focus drift? How does that work?

and the focus sensors may only accept the smaller cone of rays rather
than the full f/1.8 cone.

I don't understand any of that. :- )

It means the AF sensor has a smaller effective aperture than is being
used to take the photograph. If you want the best results from a wide
aperture 50mm of spherical design then you need to understand
this.

What do I have to do differently because of it?

Alternatively you could buy a modern aspherical design. One of
the great benefits of modern technology is that it enables people who
don't understand what they're doing to do it well.

I think that often cuts both ways. :- )

Paul Furman wrote:
Wilba wrote:
Paul Furman wrote:

OK, here's a possible explanation for the front/back difference. The
character of the out of focus area in front & back is indeed different.
In terms of bokeh, the background is usually smoother and the foreground
harsh. There are a few specialized 'defocus control' lenses which have
an extra ring to change this relationship. I'm not sure quite how that
would effect the AF confirm but it could be a factor.

Very interesting idea, thanks very much. This -
http://www.stacken.kth.se/~maxz/defocuscontrol/ (and the links in it to
http://www.luminous-landscape.com/essays/bokeh.shtml) - gave me enough
to get what you're saying.

That brings us back again to the notion that it is the optical
performance of the 50/1.8 that causes the erratic focus from the farside,
not the perceived crudeness of the focus mechanism.

What's different for me now is that I believe the erratic farside focus
is not about the sharpness wide open, but more likely about the different
"character of the out of focus area" on the nearside and farside of the
plane of focus.

Yeah, that's the only thing that makes sense, though it's still a bit
sketchy. In the middle of this page:
http://www.stacken.kth.se/~maxz/defocuscontrol/
the crops of dark blurry blobs show harsh rimmed OOF circles and perhaps
the AF sensor is grabbing those edges. I don't think that's quite right
but the best I can come up with.

It's a credible idea. :- )

Thanks a bunch for your help!

Paul Furman wrote:
Chris Malcolm wrote:
Wilba wrote:
David J Taylor wrote:
Wilba wrote:

Any thoughts about why the 50/1.8 figures make sense (farside focus...
0.8mm outside the DOF), but it doesn't work like that for the 18-55?

As others have said, the focal point will shift with f/number,

How is that a factor when shooting at widest apertures?

Because with a specific lens and AF sensor the focusing takes place
always at the effective aperture of the AF sensor, whereas the
aperture with which the photograph is taken can vary. Many cameras
have a max AF sensor aperture of around f6. Some go down as far as
f2.8. Hence if you're using a lens at f1.8, and it happens to be a
spherical lens design with aperture related focus drift (as many of
the golden oldie 50mms are), then this is an important factor.

Yeah, this could still be the explanation. Which direction does the focus
shift when stopping down? -that would provide a clue.
http://diglloyd.com/diglloyd/free/FocusShift/index.html
"With every lens I’ve tested to date, the focus moves farther away. For
example, if focus at f/1.4 is centered at 1.00 meters, then by f/2.8 it
might now be centered at 1.02 meters."

On average (ignoring front/back approach), Wilba's camera is focusing
closer (when 'stopped down' through obstructions to the AF sensor),
then opening fully to the sensor for taking the pic; the actual point in
focus is further back. That's the opposite of what this effect would
suggest.

My version of this jigsaw puzzle has several significant pieces missing.
:- )

"Obstructions to the AF sensor" - what is that? How is that "stopped down"
(for the AF sensor?)?

What part of the camera+lens system is "opening fully to the sensor for
taking the pic"? That sounds like something you'd say about the aperture,
but that isn't closing at any stage so it can't then open...?

Completely not getting what your saying. :- )

David J Taylor wrote:
Wilba wrote:
David J Taylor wrote:
Wilba wrote:

Any thoughts about why the 50/1.8 figures make sense (farside focus...
0.8mm outside the DOF), but it doesn't work like that for the 18-55?

As others have said, the focal point will shift with f/number,

How is that a factor when shooting at widest apertures?

and the focus sensors may only accept the smaller cone of rays rather
than the full f/1.8 cone.

I don't understand any of that. :- )

See the ray diagram he
http://en.wikipedia.org/wiki/Spherical_aberration

With spherical aberration, for example, rays from the edge of the lens
(i.e. full aperture) focus at a different point from those from the middle
part of the lens, so the focus "point" can shift with aperture. The focus
sensors tend to have a narrower acceptance angle than f/1.8, hence they
will adjust the lens so that rays from nearer the centre will be focussed,
leaving the outer rays focussing at a different, incorrect position.

Sure. So when the aperture *doesn't* change (focus and exposure both occur
at the same aperture), how do you get focus shift in the centre of the
image?